gitattributes: Do not export any .git paths in git archive

We have paths for `.gitattributes` itself, `.gitignore`, and
`.github`.  None should be exported in source archives.

.gitattributes

@@ -1,4 +1,4 @@
-.gitattributes   export-ignore
+.git*            export-ignore
 .hooks*          export-ignore
 
 # Custom attribute to mark sources as using our C code style.